Unnecessary merging of variables during process instance creation

[Zelldon]

Description

Related to #12035

Related to #12033
Came up in #11813

Setting variables during process instance creation

When a new process instance is created any given variables are set in the process instance scope. Therefore, it calls #mergeLocalDocument():

https://github.com/camunda/zeebe/blob/ae6dbfc5439c889ad8b8fd24d7431b46110c7900/engine/src/main/java/io/camunda/zeebe/engine/processing/variable/VariableBehavior.java#L60-L80

This iterates over the variables to set and check if a variable with the given name already exists in the scope:

https://github.com/camunda/zeebe/blob/ae6dbfc5439c889ad8b8fd24d7431b46110c7900/engine/src/main/java/io/camunda/zeebe/engine/processing/variable/VariableBehavior.java#L186-L187

This results in unnecessary gets in RocksDB, because at that point could have known that it creates a new instance and set initially the variables. Instead, Zeebe could just set the variable without checking for any potential existing variables, see 12b4699